Coated paper obtained by applying a paper coating composition mainly comprising a pigment and an aqueous binder on paper, followed by necessary steps, such as drying, calendering, etc., has conventionally been employed widely for commercial prints, magazines, books, and so on owing to its properties, such as excellent printing effects. With an increasing demand for higher quality and a development of high-speed printing techniques, constant efforts have been made to improve the quality of coated paper. In particular, in the field of offset printing that is in a majority among various printing techniques, it is a weighty subject to improve ink receptivity and water resistance, e.g., wet pick or wet rub, under the influence of wetting water, and anti-blister property on a rotary press.
A conventional approach to the above-described subject is addition of wet strength agents and the like, such as a melamine-formaldehyde resin, a urea-formaldehyde resin, a polyamidepolyurea-formaldehyde resin, a polyamidepolyurea-formaldehyde-epihalohydrin resin, and those disclosed in Japanese Patent Publication Nos. 11667/69 and 32597/84, Japanese Patent Application (OPI) NO. 40322/83, etc. (the term "OPI" as used herein means "unexamined published application").
These wet strength agents and the like, though exhibiting the respective effects, are not satisfactory for practical use due to serious disadvantages. For example, aminoplast resins including melamine-formaldehyde resins, urea-formaldehyde resins, etc. not only cause considerable evolution of formaldehyde from coated paper during working but also virtually fail to achieve improvements in ink receptivity or anti-blister property. Moreover, the expected effect to improve water resistance becomes hard to obtain as a pH of a paper coating composition increases. Although the polyamidepolyurea-formaldehyde resin is effective on improvement of water resistance as well as ink receptivity and anti-blister property, the degree of reached improvement is still insufficient for satisfying recent requirements for higher coated paper qualities. Therefore, there is much room for further improvement of performances.